1. Field of the Invention
The present invention relates to microlenses. More specifically, the present invention relates to cylindrical microlenses for use with laser diodes and integrated optics.
This application is related to another application, entitled "Method for Fabrication of Cylindrical Microlenses of Selected Shape", by James J. Snyder and Thomas M. Baer, Ser. No. 07/591,462, filed of even date herewith.
2. Description of Related Art
A lens is an optical element that can focus or de-focus light. The most familiar types of lenses are circular; for example, a circular converging lens focuses light to a point. Such lenses are useful for many applications, such as imaging and photography. The familiar circular lens has a shape that is symmetrical around the optical axis.
Another important type of lens is a cylindrical lens. A cylindrical converging lens focuses light along a line, typically termed the "line focus". The typical cylindrical lens is shaped symmetrically around a principal axis, which is orthogonal to the optical axis. For example, a cylindrical glass lens may have the shape of a cylinder, with circular dimensions around a central axis. Light is made incident on a first curved surface of the cylinder, and exits from the other, second curved side of the cylinder.
However, for many applications, a circular cross-section is undesirable, and therefore, the curves of cylindrical lenses may require specific shapes quite different from the circular curve of the previous example. The required shape might be flat or it could be some other non-circular curve such as elliptical or hyperbolic. In other words, cylindrical lenses may be formed with a variety of curved surfaces. The exact shape chosen is highly dependent upon the application.
The circular and flat shapes are easy to manufacture, and are common in cylindrical lenses. However, these shapes have disadvantages, such as spherical aberration which causes mis-focusing of marginal rays. Spherical aberration can be substantially reduced by careful design and manufacture of the shape of the input and output surfaces. Other types of aberrations, such as coma can also be reduced by careful lens design and manufacture. If a lens is designed to substantially reduce all significant aberrations, then it may be termed "diffraction-limited". A diffraction-limited lens makes efficient use of the light it receives by providing the highest intensity at the focus.
A figure of importance for any lens is its numerical aperture. Quantitatively, the numerical aperture is given by: EQU N.A. (numerical aperture)=n sin .theta.,
where .theta. is the angular semi-aperture of the lens and n is the refractive index of the medium in which the light is focused. The numerical aperture is a measure of the resolving and light gathering power of a lens. The numerical aperture is affected by the size of the aperture and its focal length. If the numerical aperture of a lens is greater than the numerical aperture of the source that the lens is collimating, then all light from the source can be collimated. On the other hand, if the numerical aperture of the lens is less than that of the source, then some of the light emitted from the source cannot be collimated, and may be lost or directed away. If a lens has a high numerical aperture, then it may be termed "fast".
Carefully designed lens surfaces can be manufactured on large scale optics (&gt;5 mm) with large numerical apertures (0.5 or better) by conventional grinding and polishing techniques. However, for smaller scale lenses (&lt;5 mm), conventional grinding and polishing techniques are unable to produce optical quality cylindrical lenses. For small microlenses (&lt;1 mm) other techniques have been developed. Microlenses have been manufactured using photosensitive glass, graded index glass, and as computer-generated diffractive optics or kinoforms. None of these techniques has been able to produce a lens with a numerical aperture approaching 0.5 and greater.
In fabricating microlenses from photosensitive glass, a mask is first deposited on the glass, and the material outside the desired lens is exposed to light. When the glass is subsequently heated, the exposed material expands its volume, and the unexposed lens region is compressed. The compression causes the lens region to bulge, forming a simple lens.
Graded index microlenses are formed by diffusing index-changing material into glass. The diffusion process yields an index of refraction that varies smoothly from the lens center to the edge. The graded index focuses the light much as a conventional lens does.
In a binary diffractive optic or computer-generated kinoform, the surface of a glass plate is etched according to a pattern generated by computer. The etched surface is designed to diffract light to a focal point, so that it performs like a conventional lens.
Cylindrical microlenses fabricated from photosensitive glass and graded index planar microlenses can be produced inexpensively in quantity, but these single optical elements are limited in speed to numerical apertures of 0.25 to 0.32, and furthermore they cannot be corrected for spherical aberration. Diffractive optic kinoforms can be corrected for aberrations, but efficient kinoform lenses with numerical apertures approaching 0.5 require the use of sub-quarter-micron lithography, which is currently beyond the state of the art.
Optical fibers with a circular cross-section have been used for cylindrical lenses. Optical fiber is inexpensive and readily available. However, circular optical fibers are not corrected for spherical aberration; i.e., such optical fibers are not diffraction limited.
It would be an advantage to provide a custom-designed, diffraction limited, fast cylindrical lens, and an inexpensive method for making a such a lens. The lens could be designed with any of a variety of input and output surfaces. Such a lens could be designed to correct for spherical aberration, for example.
Cylindrical microlenses could be utilized for integrated optics, and for focusing of laser diode bars. In integrated optics, a carefully designed cylindrical microlens could efficiently and conveniently couple light into or out of narrow waveguides, or any narrow slit.
In another application, cylindrical microlenses could form a part of a low cost, high efficiency laser diode system for pumping higher power lasers. Presently, high power lasers have a gain material that is optically pumped by high intensity flashlamps that are inefficient and have high voltage requirements. Compared with flashlamps, laser diodes are more efficient and long-lived, and require low voltage electrical sources rather than the high voltage sources used to pump flashlamps. Replacement of flashlamps with laser diodes would increase efficiency of a high power laser by reducing electrical costs, and such replacement would also increase reliability and longevity. Furthermore, a laser diode emits substantially a single wavelength, which can be chosen to match the absorption spectra of the gain material for high efficiency conversion from pump energy to stored energy in the gain material. The pumping energy can be supplied from an array of laser diodes, which may comprise a number of laser diode bars closely stacked. In such an arrangement, it is useful if substantially all the light emitted by the laser diode bars is delivered to the solid state gain material. For this purpose, it is advantageous that the diode laser beams from each individual laser diode bar be directed to the gain material. Any portion of the beam not directed to the gain material may be lost energy. However, the laser diode bars have a numerical aperture of about 0.5, and therefore a suitable cylindrical lens should have a 0.5 numerical aperture or higher, a figure that is beyond the state of the current technology.
If one were available, a diffraction limited cylindrical lens having a numerical aperture greater than 0.5 could collimate a beam from a laser diode. A collimated beam is one that is neither converging nor diverging; i.e., the rays within the beam are travelling substantially parallel. By comparison, a focused beam converges to the point of focus, and then diverges to infinity. The laser diode bar is an efficient source of laser radiation, however the highly divergent beam emitted from the laser diode presents problems in applications. The divergence of the laser diode's beam is caused by its exit aperture, which is very narrow along one axis (the "fast" axis), and much wider along the other (perpendicular) axis. The cross-section of the beam emitted along the fast axis (the narrow aperture) is highly divergent due to diffraction effects. In comparison, the wider aperture emits a beam cross-section that diverges only slightly. Conventional optical fibers with a circular cross-section have been used to collimate the beam from a laser diode bar. However, the circular fiber is not diffraction limited; the circular shape has the disadvantage of spherical aberration and thus a large portion of the light focused by such a fiber would be misdirected.